This invention relates to a lithium ion conductive glass-ceramics suitable for use as wholly solid electric cells, gas sensors and electrochemical devices of various types, and electric cells and gas sensors using such glass-ceramics.
Recent development in electronics has brought about high-performance electronic devices of a compact and light-weight design and, as a power source of such electronic devices, development of an electric cell of a high energy density and a long life is strongly desired for.
Lithium has the highest oxidation-reduction potential of Li/Li.sup.+ of all metal elements and has the smallest mass per 1 mol and, therefore, lithium cell can provide a higher energy density than other types of cells. Moreover, if a lithium ion conductive solid electrolyte is used, this electrolyte can be made very thin and, therefore, a cell of a thin film can be formed and increase in energy density per unit volume can thereby be realized.
A lithium ion cell which has been realized to date uses an organic electrolyte solution as its electrolyte and this makes it difficult to achieve a cell of a compact design such as a thin film design. This lithium ion cell has additional disadvantages that it has likelihood of leakage of electrolyte solution and likelihood of spontaneous combustion. If this lithium ion cell is replaced by a cell employing an inorganic solid electrolyte, a wholly solid cell of a high reliability will be realized.
Moreover, carbon dioxide gas produced by combustion of fossil fuel is a main cause of a hothouse effect which has recently become a serious problem and it has become necessary to incessantly watch the concentration of carbon dioxide gas. Therefore, establishment of a system for detecting carbon dioxide gas is a matter of increasing importance for the maintenance of a comfortable life in the future human society.
Carbon dioxide gas detection systems which are currently in use are generally of a type utilizing absorption of infrared ray. These systems however are large and costly and besides are very susceptible to contamination. For these reasons, studies have recently been actively made to develop a compact carbon dioxide gas sensor using a solid electrolyte. Particularly, many reports have been made about studies using a lithium ion solid electrolyte.
For realizing such gas sensor using solid electrolyte, development of a solid electrolyte which is highly conductive, chemically stable and sufficiently heat proof is indispensable.
Among known electrolyes, Li.sub.3 N single crystal (Applied Physics letter, 30(1977) P621-22), LiI--Li.sub.2 S--P.sub.2 S.sub.5 (Solid State Ionics, 5(1981) P663), LiI--Li.sub.2 S--SiS.sub.4 (J. Solid State Chem. 69 (1987) P252) and LiI--Li.sub.2 S--B.sub.2 S.sub.3 (Mat. Res. Bull., 18(1983) 189) glasses have high conductivity of 10.sup.-3 S/cm or over. These materials, however, have the disadvantages that preparation and handling of these materials are difficult and these materials are not sufficiently heat proof. Particularly, these materials have the fatal disadvantage that decomposition voltage of these materials is so low that, when they are used for an electrolyte of a solid cell, a sufficiently high terminal voltage cannot be obtained.
An oxide lithium solid electrolyte does not have the above described disadvantages and has a decomposition voltage which is higher than 3V and, therefore, it has possibility of usage as a wholly solid lithium cell if it exhibits a high conductivity at room temperature. It is known in the art that conductivity in an oxide glass can be increased by increasing lithium ion concentration. However, there is limitation in increase in the lithium ion concentration even if rapid quenching is employed for glass formation and conductivity of this glass at room temperature is below 10.sup.-6 S/cm at the highest.
Japanese Patent Application Laid-open Publication No. Hei-8-239218 discloses a gas sensor using a thin film of a lithium ion conductive glass. The conductivity of this lithium ion conductive glass thin film is between 1.7.times.10.sup.-7 and 6.1.times.10.sup.-7 S/cm. This is not a sufficiently high value and a solid electrolyte having a higher conductivity is desired for.
There are many reports about oxide ceramics having high conductivity. For example, Li.sub.4 GeO.sub.4 --Li.sub.3 VO.sub.4 exhibits conductivity of 4.times.10.sup.-5 S/cm at room temperature (Mat. Res. Bull. 15 (1980) P1661), Li.sub.1+X M.sub.X Ti.sub.2-X (PO.sub.4).sub.3 (M=Al, Ga, Cr etc.) exhibits conductivity of 7.times.10.sup.-4 S/cm at room temperature (J. Electrochem. Soc., 137(1990) P1023) and Li.sub.1+X Al.sub.X Ge.sub.2-X (PO.sub.4).sub.3 exhibits conductivity of 3.times.10.sup.-4 S/cm at room temperature (Proceedings of 8th international meeting on lithium batteries, Jun. 6-21, 1996, Nagoya, Japan, P316-317). Oxide ceramics are superior in conductivity to oxide glasses but have the disadvantages that they require a complicated and troublesome process for manufacturing and that they are difficult to form, particularly to a thin film.
In short, the prior art lithium ion solid electrolytes have the problems that they are either low in coductivity, hard to handle, hard to form to a compact design such as a thin film.
It is, therefore, an object of the invention to provide glass-ceramics which have solved these problems and exhibit a high lithium ion conductivity at room temperature.
It is another object of the invention to provide an lithium cell and a gas sensor of a high performance by utilizing such glass-ceramics.